The major objective of this research is to study Down's syndrome infants' social competence and mastery motivation in the first year of life and to examine the competence and mastery motivation in the first year of life and to examine the relationship between the mother-infant interaction and infant's mastery behaviors. Down's syndrome infants are being studied longitudinally at 3, 6, and 8 months, and a cross-sectional sample is being studied at 12 months of age. Mastery motivation is being assessed using tasks which have been developed in this laboratory. An observational system which preserves the sequence and continuity of the transactions between mother and infant is being employed to examine the mother-infant interaction. In addition, the relationship between the infants' social and mastery behaviors and specific cognitive, temperamental, and biochemical variables is being examined.